Laser accelerometer

ABSTRACT

A bi-refringent sensor polarizes a laser beam to produce a pair of cross-polarized beams having a frequency difference proportional to the stresses applied to the sensor. The instrument measures accelerations of a movable object and the stresses applied to the sensor are substantially responsive only to accelerations in a particular sensing direction or axis. For this purpose a flexure coupling pivotally supports a stressproducing proof mass and the coupling is pivotally responsive to acceleration forces acting in the sensing direction and pivotably unresponsive to acceleration forces acting in other directions. The sensor stresses produced by the pivotal proof mass vary the index of refraction of the sensor material proportionately the acceleration so that the polarized beams produced by refringence have a frequency difference proportional to acceleration. Detection of this frequency difference provides the acceleration measurement. A magnetic field assembly reduces undesired magnetic coupling between the cross-polarized beams.

United States Patent 1 Hutchings et al.

[ LASER ACCELEROMETER [75] lnventors: Thomas J. llutchings, Orange;

Wilbur L. Zingery, Long Beach, both of Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

[22] Filed: July 2, 1973 [21] Appl. No.: 375,578

[56] References Cited UNITED STATES PATENTS 4/1970 Doyle et al 73/517 RUX 6/1970 Jacobs et al. 73/5l7 R X Primary Examiner-James J. GillAttorney, Agent, or FirmR. S. Sciascia; P. N. Critchlow Apr. 2, 1974[57] ABSTRACT A bi-refringent sensor polarizes a laser beam to produce apair of cross-polarized beams having a frequency difference proportionalto the stresses applied to the sensor. The instrument measuresaccelerations of a movable object and the stresses applied to the sensorare substantially responsive only to accelerations in a particularsensing direction or axis. For this purpose a flexure coupling pivotallysupports a stressproducing proof mass and the coupling is pivotallyresponsive to acceleration forces acting in the sensing direction andpivotably unresponsive to acceleration forces acting in otherdirections. The sensor stresses produced by the pivotal proof mass varythe index of refraction of the sensor material proportionately theacceleration so that the polarized beams produced by refringence have afrequency difference proportional to acceleration. Detection of thisfrequency difference provides the acceleration measurement. A magneticfield assembly reduces undesired magnetic coupling between thecross-polarized beams.

7 Claims, 3 Drawing Figures a $0M. [so-ra s Jl/IXTUAE /erseopyA/ESEA/SEQ EXCITA r/or/ LASER ACCELEROMETER BACKGROUND OF THE INVENTION Thepresent invention relates to laser accelerometers and, in particular, togas-laser accelerometers utilizing bi-refringent sensors to create ameasurable frequency difference representative of the accelerations tobe measured.

U.S. Pat. No. 3,517,560 Accelerometer issued June 30, 1970 to Earl D.Jacobs and Wilbur L. Zingery discloses many of the basic principlesapplicable to the use of lasers as a means of detecting and measuringaccelerations of a moving object. The disclosure of this patent notesthe fact that the frequency of oscillation of a gas laser is one of thepurest sources of electromagnetic energy available and that thisfrequency mainly is dependent upon the spacing between the mirrors whichdefine the length of the gas laser cavity. The mirrors, of course, actas optical resonators for the coherent light beam excited within thiscavity. Normally, the laser output has a particular frequency but, asthis reference notes, this frequency can be varied if the optical pathlength seen by the excited coherent light beam of the laser is varied.To produce variations in the optical path length and the outputfrequency, the disclosure considersa number of differing ways of usingstresssensitive materials as so-called acceleration sensors. Inparticular, one suggestion is the use ofa sensor formed of bi-fringentor doubly-refracting material disposed in the path of the generated gaslaser light beam to polarize this light beam into so-calledextra-ordinary and ordinary (E and O) rays, these rays seeing differentoptical path lengths so as to result in a frequency difference which canbe photoelectrically detected. If the frequency difference is maderesponsive to accelerations an instrument is provided that verysensitively measures these accelerations.

Although, the disclosure of this particular reference manifestlycontains valuable teachings applicable to the use of gas lasers asaccelerometers, it is equally clear that these teachings are primarilyof a theoretical nature rather than being concerned with the practicalimplementation of the basic principles underlying the theory. In otherwords, to the extent that the reference is presently pertinent, does itnot disclose a practical, operative laser accelerometer or a manner inwhich such an accelerometer can be used to readily measure theaccelerations of the moving or flying object.

For example, it can readily be appreciated that such an accelerometermust be exclusively responsive to particular acceleration forces actingin a so-called sensing direction, and that other forces acting indirections other than that which is being sensed be excluded so as tonot to affect the output of the instrument. This factor is notconsidered by the prior art and, in particular, it is not a matter withwhich the cited reference is in any way concerned at least insofar asthe discussion of the double defraction principle in the reference isconcerned. A further difficulty inherent in the use of thedoubly-refracting sensor is that the crosspolarized beams so producednormally are magnetically coupled. Consequently, when the optical pathdifference between these beams is small, one of them tends to quench soas to preclude any frequency difference measurement. Although thisfactor can be controlled to some extent by employing a heavily biasedbi-refringence, such bias itself creates a problem since the bias isgenerated by built-in stresses within the sensor material and thesebuilt-in stresses adversely affect the stability of the instrument. Forthis and other reasons, the use of an unbiased or isotropicbirefringentmaterial is strongly indicated, but when such materials are used, themagnetic coupling problem inherent in the cross-polarized beams inintroduced. Other implementation problems include the facts that laseraccelerometers of the type under consideration exhibit a high lock-infrequency and a high dependence on cavity frequency both of which mustbe significantly reduced to assure reliable operation. Other practicaldifficulties will become more apparent in the ensuing detaileddescription.

SUMMARY OF THE PRESENT INVENTION The present accelerometer assures thatits frequencydifference output exclusively represents accelerations in adesired sensing direction by employing a proof mass pivotally coupled tothe moving object in such a manner that the mass pivots only in responseto accelerations aligned with the desired sensing direction of theinstrument. Additionally, in the preferred form of the invention, anisotropic sensor material is employed and special means, such as aparticular magnetic field assembly, reduce the coupling between thecrosspolarized beams introduced by the bi-refringence sufficiently toassure a reliable output even when the differences in the optical pathsseen by these beams becomes very slight. Another preferred feature isthe use of a gas laser that employs a dual isotope. These features andothers will be fully discussed in the detailed description.

DETAILED DESCRIPTION OF THE DRAWINGS A preferred form of the inventionis illustrated in the accompanying drawings of which:

F IG. 1 is a perspective view showing somewhat schematically the generalcomponent arrangement used in the present accelerometer, certainportions of the components being broken away to show underlying parts;

FIG. 2 is a schematic block diagram representative of the componentarrangement of FIG. 1, and

FIG. 3 is a fragmentary portion of the sensor of the FIG. 1 showing thedesired cross-polarization introduced by its bi-refringence.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIGS. 1 and 2, thelaser of the accelerometer can be seen to include a laser body portion 1which, according to conventional practice can be formed of an ultralow-expansion fused silica material, the body portion being formed witha central cavity the length of which is defined by mirrors 2 and 3.These mirrors, of course, are the reflecting surfaces which cause anexcited gas laser beam to oscillate within the cavity at a particularfrequency which normally is determined by the cavity length. In otherwords, the optical path seen by an excited coherent laser beam is thereflective path between the mirrors and this optical path is adetermining factor in the frequency of oscillation. Mirror 2 is afull-reflection mirror while mirror'3 is partially reflective to providethe output.

The laser cavity defined by mirrors 2 and 3 is divided by a partition 4into two chambers 6 and 7, chamber 6 being an excitation chambercontaining a dual isotope gas mixture and chamber 7 being a sensorchamber that mounts a stress sensor 8 a portion of which disposed in thepath of the excited laser beam identified by numeral 9 and shown in dotand dash line in this figure. Excitation of the dual isotope mixture canbe accomplished in various manners. However, it is preferred to use aDC-excited tube for the present accelerometer and, more particularly, toemploy a plasma gain section 11 in the manner apparent in FIG. 1.However, as indicated, other excitation means, such as RF-excitation,can be employed.

Excitation chamber 6 which contains the dual isotope mixture is a closedand tightly sealed chamber preferably maintained at a reducedatmospheric pressure of 3 Torr, while chamber 7 which contains stresstensor 8 is open to environmental conditions. Manifestly, light beam 9generated by the excitation mixture in chamber 6 must travel freely fromone chamber to the other and its travel should not be affected by itspassage through partition 4 which divides the two chambers. For thispurpose a conventional antireflective window 12 is employed to transmitthe light beam and avoid as much as possible any losses due toreflections that otherwise might occur.

Another conventional component used to enhance the frequency stabilityof the laser cavity is a so-called PZT assembly 13 that monitors andmaintains the optical length of the laser cavity. As is known, thelength of the cavity may change due to thermal expansion and to assurefrequency stability it is essential to monitor and compensate for thesechanges.

Stress sensor 8 is formed of a solid, photoelastic material which is adoubly-refracting or bi-refrigent type of material well known in theoptical art. For present purposes, this material should possess goodoptical properties to the extent that it freely transmits light and,also good mechanical and thermal properties such as provide sufficientelasticity to minimize deformation and also sufficient conductivity toavoid undesirable temperature gradients. Most suitably, the sensor is anelongate, relative-thin member having a bottom and resting upon the baseof body portion 1 of the laser although the specific manner in which thesensor is supported is unimportant providing the acceleration forcesexerted along the so-called sensing axis of the instrument are appliedto the sensor. The sensing axis of the instrument shown in FIG. 1 isalong the dashed line 14 and, as seen, the central longitudinal axis ofthe sensor is aligned with this sensing axis. More specifically, theinstrument of FIG. 1 is intended to detect and measure on theaccelerations aligned with axis 14.

A proof mass 16 is used to stress the material of sensor 8proportionately with the degree of acceleration along sensing axis 14and, as will become apparent, a significant feature of the presentinvention is the fact that proof mass 16 is mounted in such a mannerthat the stresses produced by acceleration are exclusively thosestresses produced by accelerations acting along the axis 14 as opposedto acceleration forces or other forces acting along other axes of theproof mass. To achieve this desired result, proof mass 16 is flexibly orpivotally coupled to the guided missile or other moving object in whichthe accelerometer is being used. More specifically, a thin, highlyflexible sheet of metal 17 has one of its sides securely clamped bybolts 18 or the like to proof mass 16, while the other of its sides issecurely clamped by similar bolts 19 to a block-like standard 21 which,in turn, is rigidly carried by the base portion of the accelerometerinstrument. Thus, since the base instrument, as well as standard 21 maybe secured to the moving object the entire instrument, including thestandard are adapted to move with the object and experi enceacceleration forces which the object undergoes.

A significant feature of the flexible coupling is that it is so mountedthat it freely pivots only about a horizontal axis 22 which, as shown,is normal to sensing axis 14 of the instrument. In the illustratedembodiment, the restricted pivotal movement of the spring 17 is achievedby using the thin sheet metal in the form of a rectangle the corners ofwhich are bolted in the illustrated manner to both the standard andproof mass. Consequently, the spring 17 is effectively rigid so as to bepivotally unresponsive to forces tending to flex it about axes otherthan axis 22. As already indicated, the manifest purpose of flexiblecoupling is to assure that stresses created in sensor 8 are proportionalto only to forces acting along the sensing axis 14. If desired, the thinsheet spring can be fabricated in such a manner that it is pivotallyresponsive only along the desired single axis, although it does notappear that such a built in characteristic is operatively essential.Further, other pivotal mountings can be substituted such, for example,as a single-axis jewel pivot but, as is shown, this type of a mountintroduces some frictional forces which would tend to degrade theperformance. In actual practice, sheet spring 17 can be made of aluminumalthough materials which are less temperature-sensitive may be foundmore desirable. Accelerometers of the present type are relatively goodtemperature sensors so that it is highly desirable to provide materialswhich are excellent conductors and which consequently avoid as much aspossible the establishment of temperature gradients. The proof massmaterial, in addition to being a good conductor, also should be of amaterial having a very high density to permit small size and also toreduce cross-axis polarization. For these reasons, it is preferred touse such materials as Mallory or Tungsten although effective operationalso can be obtained with a lighter metal such as aluminum.

Functionally considered, proof mass 16 extends transversely over the topend of sensor 8 and is disposed in the light but firm contact with thesensor. For this reason, spring sheet 17 should have a very small springconstant along its sensing axis so that its resilience is sufficient tomaintain the contact between the sensor and the proof mass when theinstrument is not subjected to acceleration forces. Consequently, exceptfor gravitational forces which will be considered, proof mass 16normally exerts relatively negligible stressing forces upon sensor 8 andthe establishment of the stresses in sensor 8 is dependent primarilyupon acceleration forces applied along axis 14 when the moving objectincreases or decreases its speed of travel. The positive or negativeacceleration, of course, pivot the proof mass along its axis 22 to applythese stresses. Most suitably, proof mass 16 has a substantially greatercross-sectional area than the relatively thin sensor and the contactbetween the two is such as to apply a full surface loading.

Detection of the acceleration forces utilizes known principles such asthe doubly-refracting principle disclosed in the previously-referencedpatent of Jacobs and Zingery. Specifically, forces applied to sensor 8produce stresses within the material of the sensor which, because of thephotoelastic nature of the material, bi-refringently cross polarize acoherent beam of light, such as beam 9, passing through the material. Inthe present arrangement the bi-refringence introduced by the sensormaterial plane polarizes the transmitted beam into conventionalextraordinary E and ordinary O-rays oriented in such a manner that, asshown in FIG. 3, one of the rays is aligned with sensing axis 14 of theinstrument while the other ray is disposed perpendicular to this axis.Such mutually orthogonal rays or beams, as is known, see differentoptical path lengths in their travel back and forth between reflectingmirrors 2 and 3 and, as already has been stated, due to the opticaldifference between the path lengths, the crosspolarized beams have afrequency difference which can be heterodyned and measured as an outputrepresentative of acceleration. Thus, referring to FIG. 2, the output ofthe instrument is applied to a heterodyned circuit represented by block23 although, prior to application to this circuit, the output first isprocessed through an optical analyzer 24. Analyzer 24 is a planepolarizer disposed at 45 to beams E and 0 so as to pass a component ofboth of these beams in which the electrical fields are aligned. Theobvious reason for the use of such an analyzer is the fact thatheterodyning requires an input in which the electric fields are alignedand, of course, the electrical fields of beams E and 0, instead of beingaligned are disposed at right angles one to the other.

A further feature which significantly improves the operation of thepresent instrument is that sensor 8 is geneous character in which thereare no density ingredients or, in other words, no residual stresses. Asa resuit, the material is stressed only when an external force isapplied to it and its bi-refringent capacity therefore is dependent uponthe application of these external forces. Such stress-free materials canbe produced by careful thermal anealing and machining although absolutehomogenity must, for practical reasons, be considered as a theoreticalrather than a practical goal. However, it is most desirable to avoid theuse of a sensor material which has a biased bi-refringence to the extentthat it has a certain degree of built-in stress. Any such bias or, infact, the presence of any residual stresses in the material areundesirable due to the fact that the relaxing under continuous usage ofthese stresses is an uncontrollable factor which degrades the stabilityof the instrument. Further, the use of the isotropic sensor providesbetter accuracy and sensitivity.

The use of the isotropic sensor, however, requires the use of aso-called magnetic field assembly identified in FIG. 1 by numeral 26.The purpose of the magnetic field assembly 26 is to reduce the magneticcoupling cross-polarized beams E and O as shown in FIG. 3. One of theknown difficulties inherent in plane-polarized beams such as beams E andO is the fact that these two beams present a magnetic coupling to theextent that when the stresses on the sensor are light the differencebetween the beams becomes quite small and one of the beams tends toquench. Obviously, the frequency difference used to measure accelerationthen would become impossible to achieve. Biased bi-refringence whichutilizes a predetermined residual stress can offset this difficulty, butsuch bias, for reasons already explained, is a degrading factor.Alternatively, the isotropic sensor manifestly must operate under verylight stresses so that the magnetic coupling of the planepolarized beamsmust be taken into consideration. Magnetic field assembly 26functionally counteracts the inherent magnetic coupling to such anextent that a frequency difference output can be realized regard less ofthe degree of stress imposed upon the sensor. Anyfunctionally-equivalent assembly can be employed although the particularassembly used for the present instrument is one which is described infull in a printed document DR-66-L-07, this document being a paperdelivered at a 1966 Symposium on Unconventional Inertial Instrumentssponsored by the Naval Ordnance Systems Command.

Another feature of the invention is its use of dual isotope gas mixturein excitation chamber 6. It has been found that such mixtures reduce theundesired dependence of the frequency difference of the E and O beams onthe cavity frequency. A suitable mixture can be provided by a 9:1 ratioof helium and neon in which there is a mixture of Ne and NE at about a1:1 ratio.

The operation of the present invention should be relatively apparentfrom the foregoing description. When the instrument is secured to amoving object the acceleration of which is to be measured, accelerationsacting in a positive or negative correction along axis 14 cause proofmass 16 to exert more or less force upon sensor 8 and the amount offorce exerted determines the stresses established in the sensor. Inturn, the stresses introduce a bi-refringence in the laser beam toproduce the frequency difference providing the measurable output. Aswill be noted, the sensing axis of FIG. 1 is a vertical axis so that theinstrument is designed to measure accelerations in the verticaldirection. However, the instrument can also be oriented in othermannersso that the sensor, for example, is disposed in a horizontal plane inwhich, as already expressed the flexible coupling supplies sufficientforce to maintain the desired contact between the sensor and the proofmass. Also, it will be recognized that accelerometers of this type areuseful in inertial guidance applications which involve operation outsideof the gravity field of the earth. Under such conditions, the flexureagain maintains the desired sensor and proof mass contact. If theinstrument is to be used within the earths gravitational field, it isobvious that the weight of the proof sensor will exert a predetermineddegree of force on the sensor so that the sensor itself will besubjected to a certain degree of stress. However, this stress can betaken into consideration by using a scaled output and, of course, thisweight-produced stress does not involve the same difficulties as the useof a biased bi-refringence which has a built-in stress. As also will berecognized, the sensitivity of an accelerometer of this type is of suchadegree that, when used within the earths gravitational field, thealtitude of the moving object can vary the output due to the fact thatthe gravitational force varies as a square of the distance of the objectfrom the center of the earth. Again, however, such a variation can beaccommodated by the use of a scaled output.

In general the present instrument provides a practical laseraccelerometer capable of being employed in any moving object tosensitivily and accurately measure the accelerations of the object. Itis practical since it is capable of measuring only the accelerationforces aligned with a particular sensing axis which can be varied toaccommodate any desired situation or, if desired, multiple instrumentscan be employed to provide data along different sensing axes.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

We claim:

1. A laser accelerometer for measuring accelerations of a moving objectalong a particular sensing-direction axis comprising:

a body portion carried by said moving object and provided with a lasercavity,

laser mirrors disposed one at each end of the cavity for defining thecavity length,

a partition dividing the cavity into a sensor chamber and an excitationchamber,

means for generating a beam of coherent light in said excitationchamber,

window means formed in said partition for passing said beam from onechamber to the other at a particular cavity frequency,

an elongate relativelythin sensor carried by said moving object, saidsensor being formed of a solid photoelastic material and being disposedin the sensor chamber in the path of said light beam with itslongitudinal axis substantially aligned with said particularsensing-direction axis,

a plate-like proof mass extending transversely across and in flushcontact with one end portion of said elongate sensor,

flexure means carried by said moving object, said means pivotallycoupling said proof mass to said object and said coupling having apivotal axis about which said proof mass freely pivots in response toacceleration forces acting in a direction aligned with saidsensing-direction axis of the accelerometer, the other axes of thecoupling being relatively rigid for rendering said proof mass pivotallyunresponsive to acceleration forces acting along said other axes,whereby stress forces pivotally applied to said sensor by said proofmass are proportional to accelerations aligned with saidsensing-direction axis and the index of refraction of the photoelasticsensor material varies proportionally with said aligned accelerations,

said stressed sensor material bi-fringently introducing into said lasercavity a pair of spatially-coincident oscillator beams having anorthogonal polarization and having a frequency difference variable inresponse to said sensed accelerations of the accelerometer, and meansfor measuring frequency difference. 2. The accelerometer of claim 1wherein said sensor is formed of an isotropic material having minimalresidual stresses in the absence of externally-applied forces,

the accelerometer further including means for reducing the magneticcross-coupling of said oscillator beams sufficiently to preclude thequenching of one of the beams when said sensor stresses are small. 3.The accelerometer of claim 1 wherein said flexure means includes:

a rectilinear thin-sheet spring member and a standard member movablycarried by said moving object in a laterally spaced disposition relativeto said proof mass, said spring member spanning said space and beingrigidly clamped along its outer edges both to said proof mass and saidstandard, the spring having a small spring constant sufficient tomaintain a light and firm contact between the proof mass and the sensor,and said rigid clamps imparting a transverse rigidly to said lateralspan of the spring whereby the spring is flexibly-responsive almostexclusively to accelerations acting in a direction aligned with saidsensing axis of the accelerometer. 4. The accelerometer of claim 3wherein said sensor is formed of an isotropic material having minimalresidual stresses in the absence of externally applied forces,

the accelerometer further including means for reducing the magneticcross-coupling of said oscillator beams sufficiently to preclude thequenching of one of the beams when said sensor stresses are small.

5. The accelerometer of claim 4 wherein said excitation chamber isfilled with a dual isotope gas mixture capable of reducing thedependence of the said measured difference frequency on said cavityfrequency.

6. The accelerometer of claim 5 wherein said excitation chamber isfilled with a gas mixture formed ofa 9: 1 ratio of helium and neon, theneon being a dual isotope mixture of Ne and Ne of approximately a 1:1ratio.

vided by a plasma gain section.

1. A laser accelerometer for measuring accelerations of a moving objectalong a particular sensing-direction axis comprising: a body portioncarried by said moving object and provided with a laser cavity, lasermirrors disposed one at each end of the cavity for defining the cavitylength, a partition dividing the cavity into a sensor chamber and anexcitation chamber, means for generating a beam of coherent light insaid excitation chamber, window means formed in said partition forpassing said beam from one chamber to the other at a particular cavityfrequency, an elongate relatively-thin sensor carried by said movingobject, said sensor being formed of a solid photoelastic material Andbeing disposed in the sensor chamber in the path of said light beam withits longitudinal axis substantially aligned with said particularsensing-direction axis, a plate-like proof mass extending transverselyacross and in flush contact with one end portion of said elongatesensor, flexure means carried by said moving object, said meanspivotally coupling said proof mass to said object and said couplinghaving a pivotal axis about which said proof mass freely pivots inresponse to acceleration forces acting in a direction aligned with saidsensing-direction axis of the accelerometer, the other axes of thecoupling being relatively rigid for rendering said proof mass pivotallyunresponsive to acceleration forces acting along said other axes,whereby stress forces pivotally applied to said sensor by said proofmass are proportional to accelerations aligned with saidsensing-direction axis and the index of refraction of the photoelasticsensor material varies proportionally with said aligned accelerations,said stressed sensor material bi-fringently introducing into said lasercavity a pair of spatially-coincident oscillator beams having anorthogonal polarization and having a frequency difference variable inresponse to said sensed accelerations of the accelerometer, and meansfor measuring frequency difference.
 2. The accelerometer of claim 1wherein said sensor is formed of an isotropic material having minimalresidual stresses in the absence of externally-applied forces, theaccelerometer further including means for reducing the magneticcross-coupling of said oscillator beams sufficiently to preclude thequenching of one of the beams when said sensor stresses are small. 3.The accelerometer of claim 1 wherein said flexure means includes: arectilinear thin-sheet spring member and a standard member movablycarried by said moving object in a laterally spaced disposition relativeto said proof mass, said spring member spanning said space and beingrigidly clamped along its outer edges both to said proof mass and saidstandard, the spring having a small spring constant sufficient tomaintain a light and firm contact between the proof mass and the sensor,and said rigid clamps imparting a transverse rigidly to said lateralspan of the spring whereby the spring is flexibly-responsive almostexclusively to accelerations acting in a direction aligned with saidsensing axis of the accelerometer.
 4. The accelerometer of claim 3wherein said sensor is formed of an isotropic material having minimalresidual stresses in the absence of externally applied forces, theaccelerometer further including means for reducing the magneticcross-coupling of said oscillator beams sufficiently to preclude thequenching of one of the beams when said sensor stresses are small. 5.The accelerometer of claim 4 wherein said excitation chamber is filledwith a dual isotope gas mixture capable of reducing the dependence ofthe said measured difference frequency on said cavity frequency.
 6. Theaccelerometer of claim 5 wherein said excitation chamber is filled witha gas mixture formed of a 9:1 ratio of helium and neon, the neon being adual isotope mixture of Ne20 and Ne22 of approximately a 1:1 ratio. 7.The acceleration of claim 5 wherein said means for generating said beamin said excitation chamber is provided by a plasma gain section.